Table of Contents
5.5 Special Media (Unusual Influences)
This chapter deals with foreign materials and air changes that can dangerously affect engine operation, but can not easily be categorized into any of the preceding chapters. These influences include, for example, the ingestion of fuel (Fig. "Unintentional fuel feed") or insufficient oxygen in the intake air (Fig. "Surge due to lack of oxygen").
One unique damage type is overheating and/or a runaway turbine due to insufficient drainage of unburned fuel after an aborted start. If fuel collects in the turbine casing and does not drain in accordance with regulations, it can cause overheating which destroys the hot parts and/or makes the turbine rotor reach overspeed and burst. Insufficient drainage can be caused by malfunctioning of the drain system (the drain valve, etc.) or an unusual engine angle that prevents proper fuel drainage. In one reported case, a helicopter engine under development was restarted in a vertical position after an aborted start during a test run. The location of the drainage opening prevented fuel from draining when the engine was in a vertical position. When this fuel burned off, the engine reached uncontrollable overspeed and the turbine disk burst, which was a very dangerous situation for the testing rig personnel.
During an acceptance run of a mid-size engine, the operating behavior deteriorated within minutes. Unallowable performance losses and compressor surges occurred. Inspection of the engine revealed a greenish, lacquer-like coating on the compressor blading. Investigation and research followed by a damage analysis determined that painting work was being done in an open area during the acceptance run.
The paint mist traveled over about 50 meters into the 8 meter-high compressor intake chute, causing damages of several hundred thousand German Marks (disassembly, overhaul, repeated acceptance run; 2 Marks = 1 Euro). (A) source, (B) intake flow, (C) engine.
Example "Fueling" (Fig. "Unintentional fuel feed"):
When fueling up an air pressure-inducing starter unit powered by a small gas turbine, a few droplets of fuel splashed into the air intake chute when the fuel hose was removed from the tank. Reconstruction showed that the ingested fuel amount would have filled a small shot glass. Due to the short time, however, this corresponded to an extremely high fuel-throughflow rate, and the regulator was no longer able to sufficiently limit the RPM. Within seconds, the rotor accelerated to high overspeed like an expanding spring, causing the relatively heavy radial turbine disk to burst. The person who was fueling the tank was seriously injured, and a rotor fragment was found roughly 250 meters away.
Figure "Air pollution": During operation on or near the ground (taxiing, takeoff, landing), or during test and control runs in testing rigs or on the aircraft, there is a special danger of damaging foreign materials being ingested. It is difficult to imagine the long distances across which engines can suck up foreign materials from a source. The potential damaging effect of foreign media is also frequently underestimated.
The top diagram shows the effects of gaseous media and dusts that are sucked into engines, even over apparently safe distances (Example "Paint mist")
Typical examples include lacquer mists, dust created during manufacturing processes, dusts created during loading procedures in industrial zones, sprayed materials, and artificial fertilizer dusts in agricultural areas.
Fertilizer dusts: the spreading of artificial fertilizers results creates dusts with components such as sulfur and phosphorus compounds that can damage hot parts through HGC (Chapter 5.4.5).
Lacquer mists can foul (rough bonding deposits) compressor rotor blading and unallowably worsen the operating behavior of the compressor (bringing the surge limit closer and decreasing compressor efficiency). In addition, these pigments often contain heavy metal oxides that can corrosively react with the hot parts.
Dusts from industrial processes: Iron oxides, for example, tend to block tight cooling air ducts in the turbine blading.
Figure "Fire extinguishing media": The activation of fire extinguishing units takes place in the engine nacelles, the engine area, testing rigs, or on the ground (e.g. during a tailpipe fire).
In this case, various extinguishing materials are sprayed onto and/or into the engine. These materials may contain components that can cause corrosive damage in the compressor or in the hot parts during continued operation. For this reason, it is important that the extinguishing materials used are documented in order to identify engine parts that, based on their composition, could potentially suffer corrosion damage. Often, this information must be used to design an overhaul plan with corresponding cleaning and evaluation procedures.
External aggregates such as gearings typically have housings made from corrosion-sensitive light metal alloys (Al and Mg alloys) or components such as gears and roller bearings made from non-stainless steels. Even stator vane adjusters, actuators, and feedback lines can contain corrosion-sensitive steels. If the fire extinguishing material is not removed, these parts can be damaged corrosively in combination with condensation water over longer operating periods.
An important factor is whether the extinguishing material entered the engine while it was running at operating temperatures, already considerably cooled, or still hot. At high temperatures, remnants of extinguishing materials can already break down into aggressive components and react with the part surfaces. In running engines, there is the additional danger of the cooling air system carrying remnants of extinguishing material into the cooling air ducts of the turbine blading, where they can block up both the ducts and the bores of the cooling air film. In one reported case, only 10 operating hours after fire extinguishing material entered the compressor, a boroscopic inspection of the turbine rotor blades revealed considerable signs of overheating due to blocking of the cooling air ducts.
Additionally, chemical reactions can attack the part from the inside and/or worsen the heat transfer into the cooling air by creating heat-insulating oxide coatings. This creates a risk of overheating and premature failure.
Typical extinguishing materials and halons, and their potentially corrosive components (Ref. 5.5-1):
Halon 1301 contains CF3Br, i.e. the halogneides fluorine and bromium which at least temporarily create an aggressive acid during the extinguishing process. This presents at least potential danger of wet corrosion on light metals and steels. Further research is necessary to determine the extent in which titanium alloys are damaged.
Halon 1211 contains CBrClF2 , i.e. also contains Cl, the various corrosive effects of which are covered in section 5.4 (pitting corrosion, SCC, corrosion fatigue). Remnants containing Cl can also threaten titanium alloys (Fig. "Sensitization during operation").
Modern foam extinguishing systems contain NH4HSO4. After long run times, the sulfur in these deposits can cause sulfidation (Fig. "Outer sulfidation on turbine blades") in combination with alkali salts (NaCl) in marine atmospheres. If the foam strikes hot surfaces, such as those of turbine blades, glass-like deposits are created that can be removed from uncoated parts through gentle abrasive blasting (in the disassembled part). Other extinguishing foams contain up to 5% alkali sulfates and fluorine compounds (up to 5%). This means that all ingredients for sulfidation attack are present.
In one aircraft type, the tank opening was located in the inlet area of one of the engines. The fueling personnel evidently forgot to close the tank properly. This allowed the engine to continually suck fuel out of the tank opening after startup, and even cutting off the normal fuel feed was not sufficient to stop the engine. The engine showed extreme damage from a fire and unallowable overspeed.
Comment: Unfortunately details of this failure are not known. Also answer to the question, if a 'ground vortex' (Fig. "Vortex I") to the fuselage region before the aeroengine was enough for the necessary suction.
Example "De-icing" (Ref. 5.5-6 and Ref. 5.5-7):
Excerpt: “…actions follow uncontained failures of the ….APU on three …(airplanes) during ground deicing operations…In (the last)… incident uncontained fragments from the failed APU not only cause damage to the …(aircraft's) tail cone, but one fragment also penetrated the aircraft's aft pressure bulkhead and became embedded in the first aid kit stored behind the aft flight attendant's jump seat at the rear of the cabin.
In its investigation into the March 2001 incident, the NTSB found that if the …APU ingests enough Type 1 icing fluid, an ethylene glycol solution that is combustible when compressed, it will sustain combustion even if its electronic control unit senses an overspeed as a result of the ingestion and cuts off fuel to the APU.
Then, since the ECU no longer has command of the APU's rotor speed, the rotor will continue to accelerate as the icing fluid keeps burning, until the wheel bursts.
Comment: Altogether seems that 3 similar failures occurred. Obviously the unfavourable position of the APU intake duct without a liquid deflector at the fuselage upper side of this aircraft type is known. The maintenance manual demands appropriate caution during de-icing. Usually this will be considered. As deicer is used the type 1 is a flammable mixture of ethylene-glycol. The operator kept the APU running during the de-icing. This promoted the ingestion of the de-icing liquid. After this incident it was instructed to shut the APU down before de-icing.
Figure "Unintentional fuel feed": Unintentional fuel feed into the engine can be occur in various different ways:
Inside the engine:
- Media confusion between water and the water-methanol for the injection system in the compressor (Fig. "Kerosene in water injection system"). Extreme overheating damage with catastrophic engine failure is the result.
- Insufficient drainage (top diagram) of unburned fuel after an aborted engine start. The fuel collects in the combustion chamber housing and ignites during an attempted restart. This results in serious consequential damages such as fire damage on the hot parts and/or runaway turbines bursting.
Fuel entering the engine from outside:
- During fueling (left diagrams), such as on the ground as depicted here, carelessness by the fueling personnel can result in fuel remnants being ingested by the engine. Surprisingly small amounts are sufficient to cause overheating of the hot parts and/or dangerous overspeed (Example "Fueling"). The bottom diagram shows mid-air refueling during which the tank probe broke off and allowed fuel to escape from the fuel pipe.
- Ingestion of fuel by the engine (middle right diagram). This can occur if tank valves in front of the engine intake are not properly closed (Example "Open fuel tank").
- Ingestion of flammable icing fluid: Example "De-icing" describes cases in which this resulted in overspeed and bursting of the rotor of an APU turbine.
- Ingestion of fuel that was released in flight when flying through ones own “fuel cloud” (bottom right diagram). There has been at least one reported case in which fuel was released before an emergency landing, and the airplane circled around and passed back through its own fuel cloud, resulting in an explosion and crash.
Figure "Kerosene in water injection system" (Ref. 5.5-2): An aircraft accident in the early 1970s was caused by a maintenance error (Example "Kerosene in water tank"). The two engines of the affected aircraft type were outfitted with an injection unit for water into the compressor in order to increase thrust on hot days (Chapter 5.1.5). A mistake resulted in the unit being filled with kerosene. When the injection system was used during startup, the additional fuel caused extreme overheating of the turbines and both engines failed. Several people were killed during the following attempted emergency landing on a highway.
Excerpt (Ref. 5.5-2): “Cause of crash… twinjet transport on takeoff … tentatively has been laid to contamination of water used for the Rolls-Royce Spey engine injection system by kerosene.
German investigation authorities have found indications that two of the five tanks, which are supposed to contain demineralized water, contained pure kerosene. Fluid in the five tanks is pumped to an injection tank in the tail when the pilot selects water for takeoff, as was done on the ….flight when both engines failed at low altitude.
Excerpt (Ref. 5.5-4): ”…the five unmarked containers, which were stored in the cargo hold of the (aircraft) and supposedly contained “demineralized water”, were standing there…at least one container was filled with pure kerosene, a second contained a mixture of kerosene and water, and a third one was filled with pure water. In Hamburg the deadly mixture was pumped into the injection tank…a worker (apparently said)…to the copilot in the airport…“there`s fuel in there,” whereupon…(the copilot replied) “no, everything around here smells like fuel.”
Because the lighter kerosene content floated at the top of the aircraft`s water tank, the accident occurred as follows: during the startup and climbing phases, the pump (taking liquid from the bottom) first sucked up water and then pure kerosene. Experts were able to see the sudden extreme engine temperature increase in the crash fragments: deep burn marks…were found on the injection nozzles of the cooling system; the turbine blading had melted.
Comments: This extreme maintenance mistake indicates fundamental flaws in the whole quality assurance system. This applies not only to the maintenance personnel, but also the management and logistics.
The reaction of the copilot to the warning indicates insufficient risk-consciousness and mis-estimation of the technical relationships.
The damage process was surely incomprehensible at first because overheating occurred only relatively late during takeoff. This shows how important it is for specialists who are conducting investigations to have, along with their engine-specific technical knowledge, a practice-oriented broad technical education. This of course includes an understanding of physical effects and relationships.
“The first accident in 1974 attracted considerable attention because the exhaust fumes of an incineration plant caused an engine to fail. The following parameters were determined during the accident investigation.
Exhaust gas temperature = + 132 °C and exhaust gas composition = 79% nitrogen, 5% oxygen, 10% carbon dioxide, and 6% steam.
The high temperature of the gases only caused a drop in performance. The low oxygen content of 5% is the same as the oxygen content of air at 11,000 meters, where no (helicopter) engine can operate, they stop functioning. A further problem is the invisibility of these exhaust fumes; the shape and size of a visible exhaust gas cloud merely show the distribution of the steam, and not the heavier components.”
“Due to the above case, in their aviation accident information from November 1996, the aviation accident division of the national aviation authority addresses the dangers to helicopter crews during flights in the immediate vicinity of chimneys. As early as 1974, three occupants of a helicopter were killed when the engine failed during hovering flight around a chimney. Autorotation failed in this densely built-up area.”
Comment: The dropout of an aeroengine can be caused by different reactions of compressor and combustion chamber:
- Surge because the surge limit/margin is exceeded (Ill. 188.8.131.52-8). Possible causes are a too high fuel flow (high pressure in the combustion chamber) as compensation of the power drop, too low mass flow because of the low density of the warm air, deposits (profile changes, roughness) on the blading.
- Quenching/flame of the combustion chamber because lack of oxygen. Causes can be the consumption of oxygen by the fire and an insufficient compression of the compressor.
- Power drop of the aeroengine because of too low compressor performance, which can not be compensated with more fuel because of high turbine inlet temperature. A further possibility is the unsuitable reaction of the control unit at the behaviour of the aeroengine.
Figure "Potential deteriorating media from maintenance": Media from a maintenance process can influence, respectively deteriorate an aeroengine. Usually this happens if the work did not follow the specifications, respectively not approved media have been used.
In the following, typical potential deteriorations are compiled.
- (“A”), abrasive cleaning media (Ill. 19.2.3-1 and Ill. 19.2.3-2).
- Erosion of soft rub in coatings.
- Roughening of the compressor blading.
- Damage of a paint layer at the blades(aluminium, steel) from elder aeroengine types (Fig. "Delamination of coatings").
- Clogging of the cooling channels in hot parts like high pressure turbine blades and vanes.
- (“B”), washing agents (chapter 19.2.3, Ill. 19.2.3-1).
- Corrosion of sensitive coatings.
- Contamination of the lubrication oil if penetrating into a bearing chamber because of too little sealing air. This situation occurs during the washing process because of too low rotation speeds.
- Contamination of the turbine (danger of high temperature corrosion) through separating dirt from the compressor which accumulates at the turbine surfaces.
- (“C”), De-icing fluid (Example "De-icing").damages by overheating, run through of the aeroengine with fracture of the rotor.
Figure "Surge due to lack of oxygen" (Ref. 5.5-3): The operation of aircraft over fires or chimneys of power- and heating plants can result in unallowable performance decreases or, in extreme cases, flame-out due to the increased inlet air temperature and decreased oxygen supply (Example "Unmarked containers").
Interestingly, the lack of oxygen is considered to be the dominant influence (Ref. 5.5-5).
Through suitable measures (such as, for example, selectable continuous ignition and sufficient performance excesses, i.e. distance to surge limit), it is evidently possible to safely operate at relatively low altitudes over expansive and intense fires while conducting firefighting operations.
Additionally a permanent drop of the efficiency and a proneness for surging of the compressor caused from depositions at the blades (fouling) can be expected. This demands the washing of the compressor (Ill. 19.2.3-3).
A special case is the recirculation of the own exhaust gases. This is known from trust reverser operation of 4-engined airplanes and unfavourable wind regimes during stand still (Ill. 184.108.40.206-12). For vertical take-off aircrafts this situation is especially problematic (Ill. 220.127.116.11-11).
Figure "Surge due to lack of oxygen": After only several startups following an overhaul, it was noticed that the performance of the engine and the behavior of the compressor had considerably worsened. Inspection of the compressor revealed that the compressor rotor blades had a several-millimeter thick, rough, brittle coating on the pressure side and the leading edge. Microanalysis revealed that the primary components of this coating were Ca and Si. Research showed that the water used for the water-methanol injection had ten times the allowable conductivity values (a measure of water impurity). The deposits were most likely caused by injection of contaminated water into the compressor on a hot day. It is interesting to note that this amount of coating buildup could be caused by one or few injections.
Figure "Changes in aeroengine behaviour due to fouling": Deposits on the compressor blades (fouling, chapter 19.2.3) lower the compressor efficiency respectively the aeroengine performance. To compensate this, the fuel supply must be raised. This leads to increasing hot parts temperatures with shorter life time, higher repair effort/costs respectively fuel consumption and with this also costs (Fig. "Seals and engine performance").
The surge margin will be markedly smaller (Ill. 19.2.3-13) with the danger of a drop out of the aeroengine and extensive failures (Ill. 18.104.22.168-1).
The aeroengine performance drops. If this is compensated with more fuel, shortens the hot parts lifetime and this increases the fuel and repair costs.
Fouling develops, when not stocking particles like dust combine with leakage oil/oil vapour (e.g., from the front compressor main bearing “A”). Hygroscopic dust with sea salt can be itself sticky (“B”).
Also the so called “insecte roughness” in an extreme case has a similar effect. Seemingly small aeroengines with radial(!) compressors, which are already operated near the surge margin, are especially affected.
5.5.1 Measures for Avoiding Unusual Influences
- Preventing accidental fuel infeed:
- Drainage of unburned fuel remnants must be done in accordance with regulations, especially after aborted starts.
- The drainage opening must be installed so that safe fuel drainage is ensured in every allowable engine position.
- In ground aggregates, a deflector should be attached to the fuel valve neck along with a warning label with clear instructions for use. If possible, fueling should be avoided while the engines are running.
- The tank valves should be in locations that ensure that fuel cannot be ingested in case of valve leaks.
- No engine operation above larger fires:
Caution in the vicinity of fires and incinerators (see Example "Unmarked containers" and Fig. "Surge due to lack of oxygen").
If the aircraft is explicitly intended for operation over fires (e.g. for firefighting), then appropriate measures are necessary to ensure safe operating behavior of the engines (Fig. "Surge due to lack of oxygen"). For example, auto re-ignition systems are necessary (Ref. 5.5-5).
- Preventing ingestion of foreign materials:
- If possible, no operation of engines on the ground if dusts (such as fertilizers) or lacquer mistsare being released into the air in the vicinity. This is especially true for field testing rigs or test runs on the aircraft.
- Fire extinguishing materials:
- After fire extinguishing materials have entered into and/or struck the outside of an engine, the composition of the extinguishing media must be considered in order to determine which engine parts might be damaged. The compressor and hot parts should be subject to boroscopic inspections to help determine future steps. This may result in the realization that cleaning and analysis are unavoidable for estimating continued usability, at least for individual components.
5.5-1 W.Rudolph, M. Rieland, “Synthesis and Properties of Various Alternative Fire Extinguishing Agents”, AGARD-CP-587, Proceedings of the conference “Aircraft Fire Safety”, Dresden, Germany, 14-17 October 1996, Chapter 13, page 13-2.
5.5-2 “BAC 111 Crash”, periodical “Aviation Week & Space Technology, September 20, 1971, page 24.
5.5-3 “In der Nähe von Kaminen: Gefahr für Hubschrauber”, periodical “Rotorblatt” 4/96, page 41.
5.5-4 “Tödliche Mischung”, periodical “Der Spiegel”, Nr. 41, 1971, pages 182 and 184.
5.5-5 P.Gilles, “Flameout”, periodical “Aviation Maintenance”, May 2001, pages 38 to 42.
5.5-6 C. Kjelgaard, Washington DC, “FAA to ban Fokker 100&F28-4000 APU use during deicing”, “Air Transport Intelligence News”., 29 March 2002.